Re: C# Client Library Error unauthorized_client

Since you are building a service account application, please make sure you are requesting user consent involving human interaction for each Google Account that you need to access the Google My Business data from and manually authorize your app during OAuth 2.0 flow at least once prior to authenticating with the public/private key pair. You can request user consent using OAuth 2.0 installed applications flow, OAuth 2.0 web server applications flow. However, you don’t have to save your retrieved refresh tokens for a service account application. Instead, you specify the user to impersonate by specifying the email address of the user account for access to their Google My Business data when you prepare to make authorized API calls. This is because a service account is an account that belongs to your application instead of an individual end user with a Google Account.